The 22-year-old will go straight into the squad for the clash with Celtic at Parkhead on Saturday.

Manager Tommy Wright said: “He has come in on a season loan so I am pleased to get him, he ticks a lot of boxes for us.

“He has got quality and in the market we are in, it is a great option.

“He will improve the team. He is only 22, he has a presence and when you look at his record in the Championship with Morton, he scored 14 or 15 goals in 30 games or so, and we want to try to get him back to that.”

Johnstone came through the youth ranks at Celtic.

Asked what it would mean to be the one who brought the Hoops’ unbeaten record to an end, he said: “It would be massive.

“That would be the best scenario, fingers crossed.

“I had a look at the fixtures and it was a funny one when I saw it and I am delighted that hopefully I will be in the squad for that one.

“I am very much looking forward to going back there.

“Callum McGregor was in the same reserves side as I was and now he is flourishing.

“They are absolutely flying and they still have that record going and all the teams will be desperate to take that away from them as soon as possible.”

Johnstone revealed that manager Wright was “a major factor” in his decision to join the Perth club.

He said: “He called me and outlined his plans for me and it was impressive.

“I am still at the age where I can develop my game and he can be the best manager for that.

“There is nothing better than playing for a club that has good ambition and St Johnstone have definitely got that.”